Portland cement concrete, although useful in building construction, is limited in its application due to its long curing time to attain a high compressive strength. Chemical additives, such as superplasticizers and curing accelerators added to Portland cement produce high early compressive strength but are expensive.
Fly ash which is landfilled or stored in surface impoundments presents environmental concerns associated with potential soil and ground water pollution. The art has attempted to address these concerns by admixing fly ash with Portland cement during production of concrete as a means to dispose of the fly ash.
P. Kumar Mehta et al., Concrete Structure, Properties and Materials, pages 269-271, (1986) teaches that fly ash can be mixed with Portland cement to improve the strength and chemical resistance of Portland cement concrete. Mixing fly ashes such as Class F and Class C fly ash with Portland cement has gained limited acceptance, however, because the low pozzolanic reactivity of fly ash tends to increase the setting time of the Portland cement concrete.
Setting time of fly ash Portland cement blends, however, can be shortened by alkali silicates. For example, Talling, B., "Effect of Curing Conditions on Alkali-Activated Slags", Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, edited by V. M. Malhotra, pages 1485-1500, (1989), indicates that sodium silicate increases the rate of strength development in blast furnace slags.
J. Wastiels et al. "Mineral Polymer Based on Fly Ash", Proceedings of the 9th International Conference on Solid Waste Management, Widener University, Philadelphia, Pa. (1993), shows mixing fly ash and aqueous sodium silicate wherein the ratio of SiO.sub.2 :Na.sub.2 O in the silicate solution is at least about 0.80:1. Although these compositions can be heat cured, the need for very large amounts of aqueous sodium silicate solutions which have very high SiO.sub.2 :Na.sub.2 O ratios renders them slow curing.
A need therefore continues for cementitious materials which can rapidly achieve high compressive strength in shortened time periods.